This invention relates to a process of manufacturing a light alloy piston comprising an annular cooling passage in its head portion, in which a permanent mold, which preferably comprises a steel shell and which has a water-cooled bottom, is filled with the molten piston alloy, core elements, particularly a salt core for forming the cooling passage, are introduced into the molten material by means of a holder, and the permanent mold is held at an elevated temperature from the outside and is lowered into a water bath in accordance with a time program.
Cast pistons have an extremely wide field of application because the casting operation provides for a large latitude in design and because the alloy can be freely selected. In some cases, however, a pressed piston affords a higher protection against troubles which may arise in the operation of the engine. For this reason the use of pressed pistons is recommendable in racing engines, aircraft engines, engines having a large number of cylinders, and generally subjected to high mechanical and thermal stresses. Pressed pistons have a metallic structure which is dense and which in dependence on the degree of deformation is fine-grained even in thick cross-sections. The tensile strength, the yield point, the fatigue strength under repeated bending stresses, and the elongation at break are higher at temperatures between room temperature and about 250.degree. C than with cast material. For this reason the adverse effects resulting from the use of an insufficiently large piston pin are not so strong and do not arise as soon as with a cast piston.
Pressed pistons are made from light alloys, which are usually cast as bar stock with direct water chilling. Discs having the desired weight are sawn off and are heated to about 500.degree. C and then pressed to form a piston by means of a punch and die.
It is also known to make the piston blanks to be pressed in that a simple permanent mold, which preferably comprises a steel shell and which has a water-cooled bottom, is filled with a molten piston alloy, the molten material is held at an elevated temperature by gas burners directed to the chilled mold from the outside, and the mold is then lowered into a water bath in accordance with a time program. As a result, the solidification proceeds strictly from bottom to top at a predetermined velocity (Metallgesellschaft AG, Mitt. Arbeitsbereich N.F., No. 15, 1972, page 55).
Particularly in connection with relatively large pressed pistons in engines having a high power-to-weight ratio, long heat-conducting paths between the heat-receiving piston head and the ring area and the piston skirt result in excessively high temperatures at the piston head and in the ring area. This may give rise to incipient cracks or, depending on the design, to a coking of the oil or an excessively heavy wear in the annular grooves. Besides, large fluctuations of the clearances, with serious consequences, and a lower volumetric efficiency, resulting in a lower efficiency of the engine, must be expected. In such case an additional cooling of the piston with oil is essential.
An intense cooling may be effected with an annular cooling passage which is provided in the piston head and through which cooling oil is forced.
To provide such cooling passage, annular hollow cores of steel or copper or salt cores having a solid cross-section are inserted into the mold and are secured therein by means of retaining pins. After the blank consisting substantially of a solid cylinder has been cast and has then been hot-pressed to form a hollow piston, the hollow cores are dissolved out, e.g., with nitric acid, and the salt cores are flushed out with water when the inlet opening for the flow of cooling oil into the cooling passage has been formed by machining. Before the hot pressing to form the hollow piston, the pins which retain the cores are pulled out and the resulting holes are closed with cylindrical light alloy plugs (German Pat. No. 1,577,099) or with light alloy powder (Printed German Application 1,627,757). During the hot pressing of the hollow piston, the light alloy contained in the holes bonds to the piston material.